1. Field of the Invention
This invention relates to data communication, network address translators, and domain name servers; and more particularly, to the dynamic allocation of a shared network address by a network address translator to a private network device in response to a domain name server query.
2. Description of the Prior Art
The internet and the World Wide Web are rapidly expanding, with the number of new devices being connected at a phenomenal rate. A direct result of this expansion is a shortage of Internet Protocol (IP) addresses. Internet Protocol is the fundamental protocol used to route traffic across the Internet. It is typical to assign a globally unique address to each host attached to the Internet that use TCP/IP. However, in order to extend the life of the current IP addressing scheme (i.e., IPv4), address registries are requiring more justification before an organization can acquire additional IP address. Thus, an organization may not have enough assigned globally unique IP addresses to dedicate one to each host computer desiring global connectivity.
Network address translation (NAT) was developed as a way of addressing IP address depletion and scaling in routing problems. NAT allows the use of IP network addresses within a private enterprise network which are unique within the private enterprise network, but which are ambiguous outside the private enterprise network (e.g., across the Internet). This allows the same IP network number to be used in other local or private domains (i.e., private enterprise networks); and thus, helps to lessen the impact of the IP depletion problem.
For example, private enterprise networks can number their hosts according to the methodology proposed in “Address Allocation for Private Internets”, RFC 1918. A NAT router is placed at the border of the private enterprise network and is used as an interface to the Internet or other external network. The NAT router allows a host within a private enterprise to communicate with another host on the Internet (i.e., outside the private enterprise network) by translating the IP address of the private enterprise host to and from a globally unique IP address. To allow a host within the private network to be accessed by a host external to the private network, the NAT address translation must be known prior, and be statically defined. The NAT router can then use this static address translation to translate the predetermined globally unique IP address to the private network address of the host. This NAT approach works well when the number of hosts desiring global connectivity is equal to or less than the number of globally unique IP numbers assigned to the NAT router. Network address translation and it use is further described in “The IP Network Address Translator (NAT)”, RFC 1631.
When the number of hosts desiring global Internet connectivity exceed the number of globally unique IP addresses assigned to a NAT router, a NAT router implementation allowing dynamic address translation is required. Dynamic address translation allows a single globally unique IP address to be time-shared by a plurality of private network hosts. In this configuration, a NAT router assigns a private network host one of a pool of unique IP addresses to use for a finite amount of time. After a time-out period, typically based on a predetermined time period of inactivity, the globally unique IP address is relinquished, returned to the pool of globally unique IP addresses, and made available for other private enterprise hosts to use. However, current dynamic address translation NAT implementation provide for only a one-way assignment of a globally unique IP address to a private network host, which is based on a request from a host within the NAT's private enterprise network. Generally, a host external to the private network cannot reach a desired host within the private enterprise network where the NAT router uses dynamic IP address assignment. Such externally requested connectivity is only possible in the limited case where a NAT dynamically assigned IP address still resides for the desired private network host from a previous request initiated by a private network host. Once this address translation entry is removed from the NAT router's translation tables, externally-initiated connectivity is not possible. Moreover, a reason for using dynamic provisioning of globally unique IP addresses is so a plurality of private network hosts can share a limited number of globally unique IP addresses, and thus, a private network host is typically not assigned the same globally unique IP address each time.